Lightly coat the slow cooker insert with nonstick spray if desired.

Add the sliced deli corned beef, separating some slices so the soup can settle between them.

In a small bowl, whisk together the condensed French onion soup, beef broth, and Dijon mustard until smooth.

Pour the soup mixture evenly over the corned beef, ensuring the slices are well coated.

Cover and cook on low for 2 to 3 hours, just until the meat is heated through and tender, stirring once halfway through if convenient.

Gently stir before serving, then spoon the corned beef and onion gravy over mashed potatoes, noodles, or rice.